In a typical build-up wiring substrate, wiring layers and insulation layers are alternately stacked, and via holes extend through the insulation layers and connect the wiring layers to each other. Such a wiring substrate includes an insulation layer in which a reinforcement material formed by a given fiber is impregnated with an insulative resin (refer to International Patent Publication No. 2005/013653 and Japanese Laid-Open Patent Publication No. 2007-103605).
The via holes described above are formed by irradiating the insulation layer including the reinforcement material with laser beams. When the via holes are formed in this manner, the reinforcement material may project from wall surfaces of the via holes. Projection of the reinforcement material in a via hole may form a void in the via wiring. This may lower the connection reliability.